Fixed group-to-group scattering reaction rates.

- the setNumBins(..) function seems to be called multiple times,
  so a flag is implemented such that the expansion of _num_bins
  for group-to-group scattering tallying purpose is not done
  twice.
- Also in tallying the neutrons, I accidentaly had before that
  tallying only occur when the generated index is between 0 and
  # of real energy groups, which results in zeros of any non-
  thermal groups.